In modern urban areas, noise due to human activities is everywhere: noisy neighbours, construction sites, highways, train, planes, etc. And it’s not just a nuisance: it’s dangerous. A report from the European Environment Agency, which found that 125 million on the continent live in noisy environments, warns that in Europe each year there are 10,000 noise-related premature deaths. So why isn’t more done, and why does tranquillity seen to be a luxury affordable only to the privileged few?
The main reason is that up until now, there was very little that could be done: noise, as sound waves, can pass through walls very easily. Blocking noise has always required thick or heavy walls. This is dictated by something called mass density law, which holds for any sound-blocking mechanism and is based on volume mass and friction. Figure 1 illustrates the relation between the weight per area of a wall needed to effectively block noise (say a 40 dB reduction in noise level) versus sound frequency. Within the frequency range from 100 Hz to 1,500 Hz where human hearing is most sensitive, a one-layer brick wall can block noise above 350 Hz, while a one-inch gypsum board can block noise above 500 Hz. To block noise at 100 Hz, existing technologies would require the equivalent of about eight layers of bricks.
Figure 1: Weight per area of a wall needed to block noise versus sound frequency
In high-population density cities like Hong Kong, one-inch gypsum boards are commonly used as partition walls because of the high cost of living space. As a result, noise – especially low-frequency noise generated by human footsteps, music, heavy vehicle on highways, trains, air ventilation systems, construction work, factories, and power plants – is slowly eroding the well-being of those living and working in noise-polluted environments.
Since the first inception of locally resonant acoustic metamaterials in 2000, we have established a noise and vibration abatement technology platform. We are now on the verge of large-scale commercialization of products that could fundamentally change the noise abatement industry, because of their light weight, compactness, efficiency and affordability. The main functional components of the metamaterials are the decorated membrane resonators (DMR). They come in both a simple and a more complicated form. With the latter, we can make panels that are low-transmission, low-reflection or dampers for structural vibration.
The noise shield panel is probably the most relevant product for those living in noisy urban environments. At just 1 cm thick, it is significantly lighter than a one-layer brick wall and can be installed on existing walks in homes or hotels. As walls are already quite good at blocking high-frequency noise, they are instead designed to block low-frequency noise. Combined with existing walls, they can help reduce noise by over 40 dB in the entire 100 to 1500 Hz range.
Another product that could help tackle the problem of noise pollution is a stand-alone panel designed to mimic the human hearing curve and provide A-averaged transmission loss of 35 dB or more. These flexible noise shields can be rolled up and down, like curtains, and can be used to cover noisy power generators and air compressors on construction sites, to build tents for noisy subway repair crew working at night, and curtains for construction sites.
In most urban settings, noise is unavoidable. But thanks to technological advances, people now have viable, practical and affordable solutions to deal with the worst aspects of noise pollution.
Author: Yang Zhiyu, Professor, The Hong Kong University of Science and Technology.
Image: A Gulf Air jet arrives over the top of houses to land at Heathrow Airport in west London August 28, 2012. REUTERS/Stefan Wermuth